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man:clone

CLONE(2) Linux Programmer's Manual CLONE(2)

NAME

     clone, __clone2 - create a child process

SYNOPSIS

     /* Prototype for the glibc wrapper function */
     #define _GNU_SOURCE
     #include <sched.h>
     int clone(int (*fn)(void *), void *child_stack,
               int flags, void *arg, ...
               /* pid_t *ptid, void *newtls, pid_t *ctid */ );
     /* For the prototype of the raw system call, see NOTES */

DESCRIPTION

     clone() creates a new process, in a manner similar to fork(2).
     This  page  describes  both  the glibc clone() wrapper function and the
     underlying system call on which it is based.  The main  text  describes
     the  wrapper  function;  the  differences  for  the raw system call are
     described toward the end of this page.
     Unlike fork(2), clone() allows the child process to share parts of  its
     execution context with the calling process, such as the virtual address
     space, the table of file descriptors, and the table of signal handlers.
     (Note  that on this manual page, "calling process" normally corresponds
     to "parent process".  But see the description of CLONE_PARENT below.)
     One use of clone() is to implement threads: multiple flows  of  control
     in a program that run concurrently in a shared address space.
     When  the child process is created with clone(), it commences execution
     by calling the function pointed to by the argument fn.   (This  differs
     from  fork(2), where execution continues in the child from the point of
     the fork(2) call.)  The arg argument is passed as the argument  of  the
     function fn.
     When  the  fn(arg) function returns, the child process terminates.  The
     integer returned by fn is the exit status for the child  process.   The
     child process may also terminate explicitly by calling exit(2) or after
     receiving a fatal signal.
     The child_stack argument specifies the location of the  stack  used  by
     the  child process.  Since the child and calling process may share mem-
     ory, it is not possible for the child process to execute  in  the  same
     stack  as  the calling process.  The calling process must therefore set
     up memory space for the child stack and pass a pointer to this space to
     clone().  Stacks grow downward on all processors that run Linux (except
     the HP PA processors), so child_stack usually  points  to  the  topmost
     address of the memory space set up for the child stack.
     The  low  byte  of  flags contains the number of the termination signal
     sent to the parent when the child dies.  If this signal is specified as
     anything  other  than SIGCHLD, then the parent process must specify the
     __WALL or __WCLONE options when waiting for the child with wait(2).  If
     no  signal  is  specified, then the parent process is not signaled when
     the child terminates.
     flags may also be bitwise-ORed with zero or more of the following  con-
     stants,  in order to specify what is shared between the calling process
     and the child process:
     CLONE_CHILD_CLEARTID (since Linux 2.5.49)
            Clear (zero) the child thread ID at the location ctid  in  child
            memory  when  the  child  exits, and do a wakeup on the futex at
            that address.  The  address  involved  may  be  changed  by  the
            set_tid_address(2)  system  call.   This  is  used  by threading
            libraries.
     CLONE_CHILD_SETTID (since Linux 2.5.49)
            Store the child thread ID at the location ctid  in  the  child's
            memory.   The  store  operation completes before clone() returns
            control to user space.
     CLONE_FILES (since Linux 2.0)
            If CLONE_FILES is set, the calling process and the child process
            share  the same file descriptor table.  Any file descriptor cre-
            ated by the calling process or by  the  child  process  is  also
            valid  in the other process.  Similarly, if one of the processes
            closes a file descriptor, or changes its associated flags (using
            the  fcntl(2)  F_SETFD  operation),  the  other  process is also
            affected.  If a process sharing a file  descriptor  table  calls
            execve(2), its file descriptor table is duplicated (unshared).
            If  CLONE_FILES is not set, the child process inherits a copy of
            all file descriptors opened in the calling process at  the  time
            of  clone().   Subsequent  operations  that  open  or close file
            descriptors, or  change  file  descriptor  flags,  performed  by
            either  the  calling  process or the child process do not affect
            the other process.  Note,  however,  that  the  duplicated  file
            descriptors  in  the  child refer to the same open file descrip-
            tions as the  corresponding  file  descriptors  in  the  calling
            process,  and thus share file offsets and file status flags (see
            open(2)).
     CLONE_FS (since Linux 2.0)
            If CLONE_FS is set, the caller and the child process  share  the
            same  filesystem  information.   This  includes  the root of the
            filesystem, the current working directory, and the  umask.   Any
            call  to chroot(2), chdir(2), or umask(2) performed by the call-
            ing process or the child process also affects the other process.
            If CLONE_FS is not set, the child process works on a copy of the
            filesystem information of the calling process at the time of the
            clone()  call.   Calls  to chroot(2), chdir(2), or umask(2) per-
            formed later by one of the processes do  not  affect  the  other
            process.
     CLONE_IO (since Linux 2.6.25)
            If  CLONE_IO  is set, then the new process shares an I/O context
            with the calling process.  If this flag is  not  set,  then  (as
            with fork(2)) the new process has its own I/O context.
            The  I/O  context  is the I/O scope of the disk scheduler (i.e.,
            what the I/O scheduler uses to model scheduling of  a  process's
            I/O).  If processes share the same I/O context, they are treated
            as one by the I/O scheduler.  As  a  consequence,  they  get  to
            share  disk  time.   For  some  I/O schedulers, if two processes
            share an I/O context, they will be allowed to  interleave  their
            disk  access.  If several threads are doing I/O on behalf of the
            same process (aio_read(3), for  instance),  they  should  employ
            CLONE_IO to get better I/O performance.
            If  the  kernel  is not configured with the CONFIG_BLOCK option,
            this flag is a no-op.
     CLONE_NEWCGROUP (since Linux 4.6)
            Create the process in a new cgroup namespace.  If this  flag  is
            not  set,  then  (as with fork(2)) the process is created in the
            same cgroup namespaces as the calling  process.   This  flag  is
            intended for the implementation of containers.
            For  further information on cgroup namespaces, see cgroup_names-
            paces(7).
            Only a privileged process (CAP_SYS_ADMIN) can employ CLONE_NEWC-
            GROUP.
     CLONE_NEWIPC (since Linux 2.6.19)
            If  CLONE_NEWIPC  is  set,  then create the process in a new IPC
            namespace.  If this flag is not set, then (as with fork(2)), the
            process  is  created  in  the  same IPC namespace as the calling
            process.  This flag is intended for the implementation  of  con-
            tainers.
            An  IPC  namespace  provides  an  isolated  view of System V IPC
            objects (see svipc(7)) and (since Linux  2.6.30)  POSIX  message
            queues (see mq_overview(7)).  The common characteristic of these
            IPC mechanisms is that IPC objects are identified by  mechanisms
            other than filesystem pathnames.
            Objects  created  in  an  IPC namespace are visible to all other
            processes that are members of that namespace, but are not  visi-
            ble to processes in other IPC namespaces.
            When  an IPC namespace is destroyed (i.e., when the last process
            that is a member of the namespace terminates), all  IPC  objects
            in the namespace are automatically destroyed.
            Only   a   privileged   process   (CAP_SYS_ADMIN)   can   employ
            CLONE_NEWIPC.  This flag can't be specified in conjunction  with
            CLONE_SYSVSEM.
            For further information on IPC namespaces, see namespaces(7).
     CLONE_NEWNET (since Linux 2.6.24)
            (The  implementation  of  this  flag was completed only by about
            kernel version 2.6.29.)
            If CLONE_NEWNET is set, then create the process in a new network
            namespace.   If this flag is not set, then (as with fork(2)) the
            process is created in the same network namespace as the  calling
            process.   This  flag is intended for the implementation of con-
            tainers.
            A network namespace provides an isolated view of the  networking
            stack (network device interfaces, IPv4 and IPv6 protocol stacks,
            IP  routing  tables,   firewall   rules,   the   /proc/net   and
            /sys/class/net directory trees, sockets, etc.).  A physical net-
            work device can live in exactly one network namespace.   A  vir-
            tual network (veth(4)) device pair provides a pipe-like abstrac-
            tion that can be used to create tunnels between  network  names-
            paces,  and can be used to create a bridge to a physical network
            device in another namespace.
            When a network namespace is freed (i.e., when the  last  process
            in  the  namespace terminates), its physical network devices are
            moved back to the initial network namespace (not to  the  parent
            of the process).  For further information on network namespaces,
            see namespaces(7).
            Only   a   privileged   process   (CAP_SYS_ADMIN)   can   employ
            CLONE_NEWNET.
     CLONE_NEWNS (since Linux 2.4.19)
            If  CLONE_NEWNS  is  set,  the  cloned child is started in a new
            mount namespace, initialized with a copy of the namespace of the
            parent.   If CLONE_NEWNS is not set, the child lives in the same
            mount namespace as the parent.
            Only   a   privileged   process   (CAP_SYS_ADMIN)   can   employ
            CLONE_NEWNS.   It  is  not permitted to specify both CLONE_NEWNS
            and CLONE_FS in the same clone() call.
            For further information on mount namespaces,  see  namespaces(7)
            and mount_namespaces(7).
     CLONE_NEWPID (since Linux 2.6.24)
            If  CLONE_NEWPID  is  set,  then create the process in a new PID
            namespace.  If this flag is not set, then (as with fork(2))  the
            process  is  created  in  the  same PID namespace as the calling
            process.  This flag is intended for the implementation  of  con-
            tainers.
            For further information on PID namespaces, see namespaces(7) and
            pid_namespaces(7).
            Only a privileged process (CAP_SYS_ADMIN) can employ  CLONE_NEW-
            PID.    This   flag  can't  be  specified  in  conjunction  with
            CLONE_THREAD or CLONE_PARENT.
     CLONE_NEWUSER
            (This flag first became meaningful for clone() in Linux  2.6.23,
            the  current clone() semantics were merged in Linux 3.5, and the
            final pieces to make the user namespaces completely usable  were
            merged in Linux 3.8.)
            If  CLONE_NEWUSER  is set, then create the process in a new user
            namespace.  If this flag is not set, then (as with fork(2))  the
            process  is  created  in  the same user namespace as the calling
            process.
            Before Linux 3.8, use of CLONE_NEWUSER required that the  caller
            have three capabilities: CAP_SYS_ADMIN, CAP_SETUID, and CAP_SET-
            GID.  Starting with Linux 3.8, no privileges are needed to  cre-
            ate a user namespace.
            This flag can't be specified in conjunction with CLONE_THREAD or
            CLONE_PARENT.  For security  reasons,  CLONE_NEWUSER  cannot  be
            specified in conjunction with CLONE_FS.
            For  further  information  on user namespaces, see namespaces(7)
            and user_namespaces(7).
     CLONE_NEWUTS (since Linux 2.6.19)
            If CLONE_NEWUTS is set, then create the process  in  a  new  UTS
            namespace,  whose identifiers are initialized by duplicating the
            identifiers from the UTS namespace of the calling  process.   If
            this flag is not set, then (as with fork(2)) the process is cre-
            ated in the same UTS namespace as  the  calling  process.   This
            flag is intended for the implementation of containers.
            A  UTS namespace is the set of identifiers returned by uname(2);
            among these, the domain name and the hostname can be modified by
            setdomainname(2) and sethostname(2), respectively.  Changes made
            to the identifiers in a UTS namespace are visible to  all  other
            processes  in  the  same  namespace, but are not visible to pro-
            cesses in other UTS namespaces.
            Only   a   privileged   process   (CAP_SYS_ADMIN)   can   employ
            CLONE_NEWUTS.
            For further information on UTS namespaces, see namespaces(7).
     CLONE_PARENT (since Linux 2.3.12)
            If  CLONE_PARENT  is  set,  then the parent of the new child (as
            returned by getppid(2)) will be the same as that of the  calling
            process.
            If  CLONE_PARENT  is not set, then (as with fork(2)) the child's
            parent is the calling process.
            Note that it is the parent process, as returned  by  getppid(2),
            which  is  signaled  when  the  child  terminates,  so  that  if
            CLONE_PARENT is set, then the parent  of  the  calling  process,
            rather than the calling process itself, will be signaled.
     CLONE_PARENT_SETTID (since Linux 2.5.49)
            Store  the  child thread ID at the location ptid in the parent's
            memory.  (In Linux 2.5.32-2.5.48 there was a  flag  CLONE_SETTID
            that  did  this.)   The store operation completes before clone()
            returns control to user space.
     CLONE_PID (Linux 2.0 to 2.5.15)
            If CLONE_PID is set, the child process is created with the  same
            process ID as the calling process.  This is good for hacking the
            system, but otherwise  of  not  much  use.   From  Linux  2.3.21
            onward,  this  flag  could  be specified only by the system boot
            process (PID 0).  The flag disappeared completely from the  ker-
            nel  sources  in  Linux 2.5.16.  Since then, the kernel silently
            ignores this bit if it is specified in flags.
     CLONE_PTRACE (since Linux 2.2)
            If CLONE_PTRACE is specified, and the calling process  is  being
            traced, then trace the child also (see ptrace(2)).
     CLONE_SETTLS (since Linux 2.5.32)
            The TLS (Thread Local Storage) descriptor is set to newtls.
            The  interpretation of newtls and the resulting effect is archi-
            tecture dependent.  On x86, newtls is interpreted  as  a  struct
            user_desc *  (see  set_thread_area(2)).  On x86-64 it is the new
            value to be set for the %fs base register (see  the  ARCH_SET_FS
            argument  to  arch_prctl(2)).  On architectures with a dedicated
            TLS register, it is the new value of that register.
     CLONE_SIGHAND (since Linux 2.0)
            If CLONE_SIGHAND is set,  the  calling  process  and  the  child
            process share the same table of signal handlers.  If the calling
            process or child process calls sigaction(2) to change the behav-
            ior  associated  with  a  signal, the behavior is changed in the
            other process as well.  However, the calling process  and  child
            processes  still  have distinct signal masks and sets of pending
            signals.  So, one of them may block  or  unblock  signals  using
            sigprocmask(2) without affecting the other process.
            If  CLONE_SIGHAND  is not set, the child process inherits a copy
            of the signal handlers  of  the  calling  process  at  the  time
            clone() is called.  Calls to sigaction(2) performed later by one
            of the processes have no effect on the other process.
            Since Linux 2.6.0-test6, flags must  also  include  CLONE_VM  if
            CLONE_SIGHAND is specified
     CLONE_STOPPED (since Linux 2.6.0-test2)
            If CLONE_STOPPED is set, then the child is initially stopped (as
            though it was sent a SIGSTOP signal), and  must  be  resumed  by
            sending it a SIGCONT signal.
            This  flag  was  deprecated  from  Linux  2.6.25 onward, and was
            removed altogether in Linux  2.6.38.   Since  then,  the  kernel
            silently ignores it without error.  Starting with Linux 4.6, the
            same bit was reused for the CLONE_NEWCGROUP flag.
     CLONE_SYSVSEM (since Linux 2.5.10)
            If CLONE_SYSVSEM is set, then the child and the calling  process
            share  a  single  list of System V semaphore adjustment (semadj)
            values (see semop(2)).  In this case, the  shared  list  accumu-
            lates  semadj  values across all processes sharing the list, and
            semaphore adjustments are performed only when the  last  process
            that  is sharing the list terminates (or ceases sharing the list
            using unshare(2)).  If this flag is not set, then the child  has
            a separate semadj list that is initially empty.
     CLONE_THREAD (since Linux 2.4.0-test8)
            If  CLONE_THREAD  is set, the child is placed in the same thread
            group as the calling process.  To make the remainder of the dis-
            cussion of CLONE_THREAD more readable, the term "thread" is used
            to refer to the processes within a thread group.
            Thread groups were a feature added in Linux 2.4 to  support  the
            POSIX  threads  notion  of  a set of threads that share a single
            PID.  Internally, this shared PID is the so-called thread  group
            identifier  (TGID) for the thread group.  Since Linux 2.4, calls
            to getpid(2) return the TGID of the caller.
            The threads within a group can be distinguished by  their  (sys-
            tem-wide) unique thread IDs (TID).  A new thread's TID is avail-
            able as the function result returned to the caller  of  clone(),
            and a thread can obtain its own TID using gettid(2).
            When  a call is made to clone() without specifying CLONE_THREAD,
            then the resulting thread is placed in a new thread group  whose
            TGID is the same as the thread's TID.  This thread is the leader
            of the new thread group.
            A new thread created  with  CLONE_THREAD  has  the  same  parent
            process  as  the caller of clone() (i.e., like CLONE_PARENT), so
            that calls to getppid(2) return the same value for  all  of  the
            threads  in  a  thread group.  When a CLONE_THREAD thread termi-
            nates, the thread that created it using clone() is  not  sent  a
            SIGCHLD  (or  other  termination)  signal; nor can the status of
            such a thread be obtained using wait(2).  (The thread is said to
            be detached.)
            After  all of the threads in a thread group terminate the parent
            process of the thread group is sent a SIGCHLD (or other termina-
            tion) signal.
            If  any  of the threads in a thread group performs an execve(2),
            then all threads other than the thread group leader  are  termi-
            nated,  and  the  new  program  is  executed in the thread group
            leader.
            If one of the threads in a thread group creates  a  child  using
            fork(2),  then  any  thread  in  the  group can wait(2) for that
            child.
            Since Linux 2.5.35, flags must  also  include  CLONE_SIGHAND  if
            CLONE_THREAD   is   specified   (and   note  that,  since  Linux
            2.6.0-test6,  CLONE_SIGHAND  also  requires   CLONE_VM   to   be
            included).
            Signals  may be sent to a thread group as a whole (i.e., a TGID)
            using kill(2),  or  to  a  specific  thread  (i.e.,  TID)  using
            tgkill(2).
            Signal  dispositions  and actions are process-wide: if an unhan-
            dled signal is delivered to a thread, then it will affect  (ter-
            minate, stop, continue, be ignored in) all members of the thread
            group.
            Each thread has its own signal mask, as set  by  sigprocmask(2),
            but  signals can be pending either: for the whole process (i.e.,
            deliverable to any member of the thread group), when  sent  with
            kill(2);  or for an individual thread, when sent with tgkill(2).
            A call to sigpending(2) returns a signal set that is  the  union
            of  the  signals  pending  for the whole process and the signals
            that are pending for the calling thread.
            If kill(2) is used to send a signal to a thread group,  and  the
            thread  group  has  installed a handler for the signal, then the
            handler will be invoked in  exactly  one,  arbitrarily  selected
            member  of the thread group that has not blocked the signal.  If
            multiple threads in a group are waiting to accept the same  sig-
            nal using sigwaitinfo(2), the kernel will arbitrarily select one
            of these threads to receive a signal sent using kill(2).
     CLONE_UNTRACED (since Linux 2.5.46)
            If CLONE_UNTRACED is specified, then a  tracing  process  cannot
            force CLONE_PTRACE on this child process.
     CLONE_VFORK (since Linux 2.2)
            If  CLONE_VFORK  is set, the execution of the calling process is
            suspended until the child releases its virtual memory  resources
            via a call to execve(2) or _exit(2) (as with vfork(2)).
            If CLONE_VFORK is not set, then both the calling process and the
            child are schedulable after the call, and an application  should
            not rely on execution occurring in any particular order.
     CLONE_VM (since Linux 2.0)
            If  CLONE_VM  is  set, the calling process and the child process
            run in the same memory space.  In particular, memory writes per-
            formed  by  the calling process or by the child process are also
            visible in the other process.  Moreover, any memory  mapping  or
            unmapping  performed  with  mmap(2) or munmap(2) by the child or
            calling process also affects the other process.
            If CLONE_VM is not set, the child process  runs  in  a  separate
            copy  of  the memory space of the calling process at the time of
            clone().  Memory writes or file mappings/unmappings performed by
            one of the processes do not affect the other, as with fork(2).

NOTES

     Note  that the glibc clone() wrapper function makes some changes in the
     memory pointed to by child_stack (changes required to set the stack  up
     correctly  for the child) before invoking the clone() system call.  So,
     in cases where clone() is used to recursively create children,  do  not
     use  the  buffer  employed  for  the parent's stack as the stack of the
     child.
 C library/kernel differences
     The raw clone() system call corresponds more closely to fork(2) in that
     execution  in the child continues from the point of the call.  As such,
     the fn and arg arguments of the clone() wrapper function are omitted.
     Another difference  for  the  raw  clone()  system  call  is  that  the
     child_stack argument may be NULL, in which case the child uses a dupli-
     cate of the parent's stack.  (Copy-on-write semantics ensure  that  the
     child  gets separate copies of stack pages when either process modifies
     the stack.)  In this case, for correct operation, the  CLONE_VM  option
     should  not  be  specified.   (If  the child shares the parent's memory
     because of the use of the CLONE_VM flag, then no copy-on-write duplica-
     tion occurs and chaos is likely to result.)
     The  order  of  the  arguments also differs in the raw system call, and
     there are variations in the arguments across architectures, as detailed
     in the following paragraphs.
     The  raw  system  call interface on x86-64 and some other architectures
     (including sh, tile, and alpha) is:
         long clone(unsigned long flags, void *child_stack,              int
         *ptid, int *ctid,            unsigned long newtls);
     On  x86-32,  and  several  other common architectures (including score,
     ARM, ARM 64, PA-RISC, arc, Power PC, xtensa, and MIPS),  the  order  of
     the last two arguments is reversed:
         long  clone(unsigned  long  flags, void *child_stack,           int
         *ptid, unsigned long newtls,           int *ctid);
     On the cris and s390 architectures, the order of the  first  two  argu-
     ments is reversed:
         long  clone(void  *child_stack, unsigned long flags,            int
         *ptid, int *ctid,            unsigned long newtls);
     On the microblaze architecture, an additional argument is supplied:
         long clone(unsigned long flags, void *child_stack,              int
         stack_size,          /*  Size of stack */            int *ptid, int
         *ctid,            unsigned long newtls);
 blackfin, m68k, and sparc
     The argument-passing conventions on blackfin, m68k, and sparc are  dif-
     ferent  from  the descriptions above.  For details, see the kernel (and
     glibc) source.
 ia64
     On ia64, a different interface is used:
     int __clone2(int (*fn)(void *),
                  void *child_stack_base, size_t stack_size,
                  int flags, void *arg, ...
               /* pid_t *ptid, struct user_desc *tls, pid_t *ctid */ );
     The prototype shown above is for the glibc wrapper  function;  the  raw
     system  call interface has no fn or arg argument, and changes the order
     of the arguments so that flags is the first argument, and  tls  is  the
     last argument.
     __clone2()   operates   in   the  same  way  as  clone(),  except  that
     child_stack_base points to the lowest  address  of  the  child's  stack
     area,  and  stack_size  specifies  the  size of the stack pointed to by
     child_stack_base.
 Linux 2.4 and earlier
     In Linux 2.4 and earlier, clone() does not take  arguments  ptid,  tls,
     and ctid.

RETURN VALUE

     On  success,  the  thread  ID  of  the child process is returned in the
     caller's thread of execution.   On  failure,  -1  is  returned  in  the
     caller's  context,  no child process will be created, and errno will be
     set appropriately.

ERRORS

     EAGAIN Too many processes are already running; see fork(2).
     EINVAL CLONE_SIGHAND was specified, but CLONE_VM was not.  (Since Linux
            2.6.0-test6.)
     EINVAL CLONE_THREAD  was  specified, but CLONE_SIGHAND was not.  (Since
            Linux 2.5.35.)
     EINVAL Both CLONE_FS and CLONE_NEWNS were specified in flags.
     EINVAL (since Linux 3.9)
            Both CLONE_NEWUSER and CLONE_FS were specified in flags.
     EINVAL Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in flags.
     EINVAL One (or both) of CLONE_NEWPID or CLONE_NEWUSER and one (or both)
            of CLONE_THREAD or CLONE_PARENT were specified in flags.
     EINVAL Returned  by  the  glibc  clone()  wrapper  function  when fn or
            child_stack is specified as NULL.
     EINVAL CLONE_NEWIPC was specified in flags, but the kernel was not con-
            figured with the CONFIG_SYSVIPC and CONFIG_IPC_NS options.
     EINVAL CLONE_NEWNET was specified in flags, but the kernel was not con-
            figured with the CONFIG_NET_NS option.
     EINVAL CLONE_NEWPID was specified in flags, but the kernel was not con-
            figured with the CONFIG_PID_NS option.
     EINVAL CLONE_NEWUTS was specified in flags, but the kernel was not con-
            figured with the CONFIG_UTS option.
     EINVAL child_stack is not aligned  to  a  suitable  boundary  for  this
            architecture.   For  example,  on aarch64, child_stack must be a
            multiple of 16.
     ENOMEM Cannot allocate sufficient memory to allocate a  task  structure
            for  the  child,  or to copy those parts of the caller's context
            that need to be copied.
     ENOSPC (since Linux 3.7)
            CLONE_NEWPID was specified in flags, but the limit on the  nest-
            ing  depth  of  PID  namespaces  would  have  been exceeded; see
            pid_namespaces(7).
     ENOSPC (since Linux 4.9; beforehand EUSERS)
            CLONE_NEWUSER was specified in flags, and the call  would  cause
            the  limit  on  the  number  of  nested  user  namespaces  to be
            exceeded.  See user_namespaces(7).
            From Linux 3.11 to Linux 4.8, the error diagnosed in  this  case
            was EUSERS.
     ENOSPC (since Linux 4.9)
            One  of the values in flags specified the creation of a new user
            namespace, but doing so would have caused the limit  defined  by
            the  corresponding  file  in /proc/sys/user to be exceeded.  For
            further details, see namespaces(7).
     EPERM  CLONE_NEWCGROUP,   CLONE_NEWIPC,   CLONE_NEWNET,    CLONE_NEWNS,
            CLONE_NEWPID,  or  CLONE_NEWUTS was specified by an unprivileged
            process (process without CAP_SYS_ADMIN).
     EPERM  CLONE_PID was specified by  a  process  other  than  process  0.
            (This error occurs only on Linux 2.5.15 and earlier.)
     EPERM  CLONE_NEWUSER  was  specified in flags, but either the effective
            user ID or the effective group ID of the caller does not have  a
            mapping in the parent namespace (see user_namespaces(7)).
     EPERM (since Linux 3.9)
            CLONE_NEWUSER  was  specified  in  flags  and the caller is in a
            chroot environment (i.e., the caller's root directory  does  not
            match  the  root  directory  of  the mount namespace in which it
            resides).
     ERESTARTNOINTR (since Linux 2.6.17)
            System call was interrupted by a signal and will  be  restarted.
            (This can be seen only during a trace.)
     EUSERS (Linux 3.11 to Linux 4.8)
            CLONE_NEWUSER  was specified in flags, and the limit on the num-
            ber of nested user namespaces would be exceeded.  See  the  dis-
            cussion of the ENOSPC error above.

CONFORMING TO

     clone()  is  Linux-specific and should not be used in programs intended
     to be portable.

NOTES

     The kcmp(2) system call can be used to test whether two processes share
     various  resources  such as a file descriptor table, System V semaphore
     undo operations, or a virtual address space.
     Handlers registered using pthread_atfork(3) are not executed  during  a
     call to clone().
     In  the  Linux  2.4.x  series, CLONE_THREAD generally does not make the
     parent of the new thread the same as the parent of the calling process.
     However,  for  kernel  versions  2.4.7  to 2.4.18 the CLONE_THREAD flag
     implied the CLONE_PARENT flag (as in Linux 2.6.0 and later).
     For a while there was CLONE_DETACHED  (introduced  in  2.5.32):  parent
     wants no child-exit signal.  In Linux 2.6.2, the need to give this flag
     together with CLONE_THREAD disappeared.  This flag  is  still  defined,
     but has no effect.
     On  i386,  clone()  should not be called through vsyscall, but directly
     through int $0x80.

BUGS

     GNU C library versions 2.3.4 up to and including 2.24 contained a wrap-
     per  function  for  getpid(2)  that  performed  caching  of PIDs.  This
     caching relied on support in the glibc wrapper for clone(), but limita-
     tions  in the implementation meant that the cache was not up to date in
     some circumstances.  In particular, if a signal was  delivered  to  the
     child immediately after the clone() call, then a call to getpid(2) in a
     handler for the signal could return the  PID  of  the  calling  process
     ("the parent"), if the clone wrapper had not yet had a chance to update
     the PID cache in the child.  (This discussion ignores  the  case  where
     the  child was created using CLONE_THREAD, when getpid(2) should return
     the same value in the child and in the  process  that  called  clone(),
     since  the  caller  and  the  child  are in the same thread group.  The
     stale-cache problem also does not occur if the flags argument  includes
     CLONE_VM.)   To  get  the truth, it was sometimes necessary to use code
     such as the following:
         #include <syscall.h>
         pid_t mypid;
         mypid = syscall(SYS_getpid);
     Because of the stale-cache problem, as well as other problems noted  in
     getpid(2), the PID caching feature was removed in glibc 2.25.

EXAMPLE

     The following program demonstrates the use of clone() to create a child
     process that executes in a separate UTS namespace.  The  child  changes
     the  hostname in its UTS namespace.  Both parent and child then display
     the system hostname, making it possible to see that the  hostname  dif-
     fers  in the UTS namespaces of the parent and child.  For an example of
     the use of this program, see setns(2).
 Program source
     #define  _GNU_SOURCE  #include  <sys/wait.h>  #include  <sys/utsname.h>
     #include  <sched.h>  #include  <string.h>  #include  <stdio.h> #include
     <stdlib.h> #include <unistd.h>
     #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                             } while (0)
     static int              /* Start function for cloned  child  */  child-
     Func(void *arg) {
         struct utsname uts;
         /* Change hostname in UTS namespace of child */
         if (sethostname(arg, strlen(arg)) == -1)
             errExit("sethostname");
         /* Retrieve and display hostname */
         if (uname(&uts) == -1)
             errExit("uname");
         printf("uts.nodename in child:  %s\n", uts.nodename);
         /* Keep the namespace open for a while, by sleeping.
            This allows some experimentation--for example, another
            process might join the namespace. */
         sleep(200);
         return 0;           /* Child terminates now */ }
     #define STACK_SIZE (1024 * 1024)    /* Stack size for cloned child */
     int main(int argc, char *argv[]) {
         char *stack;                    /* Start of stack buffer */
         char *stackTop;                 /* End of stack buffer */
         pid_t pid;
         struct utsname uts;
         if (argc < 2) {
             fprintf(stderr, "Usage: %s <child-hostname>\n", argv[0]);
             exit(EXIT_SUCCESS);
         }
         /* Allocate stack for child */
         stack = malloc(STACK_SIZE);
         if (stack == NULL)
             errExit("malloc");
         stackTop = stack + STACK_SIZE;  /* Assume stack grows downward */
         /* Create child that has its own UTS namespace;
            child commences execution in childFunc() */
         pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]);
         if (pid == -1)
             errExit("clone");
         printf("clone() returned %ld\n", (long) pid);
         /* Parent falls through to here */
         sleep(1);           /* Give child time to change its hostname */
         /* Display hostname in parent's UTS namespace. This will be
            different from hostname in child's UTS namespace. */
         if (uname(&uts) == -1)
             errExit("uname");
         printf("uts.nodename in parent: %s\n", uts.nodename);
         if (waitpid(pid, NULL, 0) == -1)    /* Wait for child */
             errExit("waitpid");
         printf("child has terminated\n");
         exit(EXIT_SUCCESS); }

SEE ALSO

     fork(2),  futex(2),  getpid(2), gettid(2), kcmp(2), set_thread_area(2),
     set_tid_address(2), setns(2), tkill(2), unshare(2), wait(2),  capabili-
     ties(7), namespaces(7), pthreads(7)

COLOPHON

     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at
     https://www.kernel.org/doc/man-pages/.

Linux 2017-09-15 CLONE(2)

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